This invention relates to a sheet-like sensor for, inter alia, measuring stress distribution experienced by structural components and to a method for measuring resistances in a network or grid of resistances.
It is known in the art that certain materials exhibit a change in electrical resistance as a function of strain experienced by a material. A grid of members (e.g., copper wires) which change in resistance as a function of strain can be constructed and bonded to or integrated with a structural element (e.g., an aircraft wing) to detect the stresses experienced by the structural element. But, electrical connections must be made to each node of the grid. For large systems with many nodes, the sheer number of electrical connections becomes unwieldy as do the computations required to measure the change in resistance of all the legs between the nodes.
U.S. Pat. No. 5,650,570, incorporated herein by this reference, discloses a sheet-like sensor with amorphous iron-based alloy members woven into glass cloth layers separated by an insulating sheet and covered by synthetic rubber sheets. The members of the first cloth layer run parallel to each other and the members of the second cloth layer run parallel to each other but perpendicular to the members of the first cloth layer. One end of all the members of the first cloth layer are electrically connected to a first scanner and the other end of all of the members of the first cloth layer are electrically connected to a first impedance analyzer. One end of all of the members of the second cloth layer are electrically connected to a second scanner and the other end of all of the members of the second cloth layer are electrically connected to a second impedance analyzer. In this way, the change in resistance along the length of any member due to strain can be measured and the strain computed.
Unfortunately, the specific location of the strain experienced by the sensor cannot be detected. The same is true if a member fails: the sensor cannot identify the specific location of a failure. Moreover, the maximum strain that can be computed is limited by the failure strain of the ferromagnetic elements used which is between 0.2% and 0.4%. Finally, the method disclosed in the ""570 patent cannot accurately predict the stress distribution of a structural component since it only provides an estimate of where a force or pressure is applied.
It is therefore an object of this invention to provide improved sheet-like sensor for measuring stress distribution.
It is a further object of this invention to provide such a sensor which reduces the number of electrical connections required to filly analyze the stress experienced by a structural component.
It is a further object of this invention to provide such a sensor in which no electrical connections are required internal to the sensor to fully analyze the full stress distribution.
It is a further object of this invention to provide such a sensor which is capable of detecting the specific location of the strains experienced by the sensor.
It is a further object of this invention to provide such a sensor which is capable of identifying the specific location of a failure.
It is a further object of this invention to provide such a sensor which is able to measure strains of a higher magnitude.
It is a further object of this invention to provide such a sensor which can fully predict stress distribution.
It is a further object of this invention to provide a method of determining all of the impedances of a grid of leg impedances.
It is a further object of this invention to provide such a method useful in connection with a sheet-like sensor or in connection with analyzers of other electrical circuits.
This invention results from the realization that a better, less cumbersome, more accurate, and more useful sheet-like sensor is effected by arranging members which change resistance as a function of strain as a grid forming legs between both internal and external nodes but only connecting the resistance measurement analyzer to the boundary nodes and then determining all of the leg resistances based on the measured resistances of the legs between the boundary nodes using an iterative algorithm. In this way, the electrical interconnections between the analyzer and the internal nodes of the grid are eliminated thus seriously reducing the number of electrical interconnections required. Moreover, the specific location of any strains experienced by the sensor can be more accurately detected, the specific location of any failure can be identified, and full stress distribution of a structural member or component underlying the sensor can be predicted. In addition, by using pseudoelastic shape memory alloy material instead of ferromagnetic materials, strains of a higher magnitude can be measured. This invention also results from the realization that the algorithm used in connection with the analyzer of the sheet-like sensor can be used in other environments, e.g., for evaluating electrical circuits.
This invention features a sheet-like sensor for measuring stress distribution typically comprising a grid of members which change in resistance when subjected to strain, the members intersecting at internal nodes and intersecting at boundary nodes at the periphery of the grid defining a plurality of legs. An analyzer is electrically connected only to the boundary nodes and configured to calculate any change in resistance in all of the legs based solely on the measured resistance of the legs between the boundary nodes.
In one example, the members are copper wires. In another example, the wires are made of pseudoelastic shape memory alloy material. The grid of members may be encapsulated in an encapsulation material such as Kapton. In this way, the analyzer can be formed as a circuit integral with the encapsulation material. The grid may be in the shape of a polygon, e.g., a rectangle or a square. Other shapes and designs, however, are possible
Typically, the analyzer is configured to measure the resistances of the legs between the boundary nodes, to estimate the resistances of all of the legs, calculate the resistances of all of the legs based on the measured resistances of the legs between the boundary nodes and the estimated resistances of all of the legs, and to compare the calculated resistances of the legs between the boundary nodes with the measured resistances of the legs between the boundary nodes. Based on the comparison, a re-estimate of the resistances of all of the legs is made. Then, iterations of these steps are performed until the measured resistances of the legs between the boundary nodes converge to the calculated resistances of the legs between the boundary nodes to thus accurately determine the resistances of the legs between or connected to the internal nodes.
In one example, the analyzer is further configured to calculate the strain experience by each leg. Also, the analyzer may be further configured to identify any leg which has failed based on a very high determined resistance. Typically, the initial estimate is based on the measured resistances, e.g., the initial estimate is set to the mean of the measured resistances. Also, relaxation techniques may be used.
A sheet-like sensor for measuring stress distribution in accordance with this invention typically includes a grid of members which change in resistance when subjected to strain, the members intersecting at internal nodes and intersecting at boundary nodes at the periphery of the grid defining a plurality of legs. An analyzer is connected only to the boundary nodes. In the preferred embodiment, the analyzer is configured to measure the resistances of the legs between the boundary nodes and estimate the resistances of all of the legs, calculate the resistances of all of the legs based on the measured resistances of the legs between the boundary nodes and the estimated resistances of all of the legs. The calculated resistances of the legs between the boundary nodes is compared with the measured resistances of the legs between the boundary nodes. Based on the comparison, a re-estimate of the resistances of all of the legs is made, and iterations continue until the measured resistances of the legs between the boundary nodes converge to the calculated resistances of the legs between the boundary nodes. In this way, the resistances of the legs between or connected to the internal nodes is accurately determined.
This invention also features a sensor system or method for a grid including internal nodes and boundary nodes at the periphery of the grid defining a plurality of legs in which a characteristic of the legs between the boundary nodes is measured, the same characteristic of all of the legs is estimated, and the same characteristic of all of the legs is calculated based on the measured characteristic of the legs between the boundary nodes and the estimated characteristic of all of the legs. Next, a comparison is made between the calculated characteristic of the legs between the boundary nodes and the measured characteristic of the legs between the boundary nodes. Based on the comparison, the characteristic of all of the legs is again estimated, and iterations continue until the measured characteristic of the legs between the boundary nodes converge to the calculated characteristic of the legs between the boundary nodes. In one example, the members change in resistance when subjected to strain and the characteristic analyzed is resistance which varies as a function of strain. In one example, the characteristics are complex impedences.
One exemplary method for determining impedances in a grid of leg impedances in accordance with this invention includes: a) measuring the resistances of the legs between the boundary nodes, b) estimating the resistances of all of the legs, c) calculating the resistances of all of the legs based on the measured resistances of the legs between the boundary nodes and the estimated resistances of all of the legs, d) comparing the calculated resistances of the legs between the boundary nodes with the measured resistances of the legs between the boundary nodes, e) based on the comparison, re-estimating the resistances of all of the legs, and f) iteratively repeating steps c)-e) until the measured resistances of the legs between the boundary nodes converge to the calculated resistances of the legs between the boundary nodes to thus accurately determine the resistance of the legs between or connected to the internal nodes. Further included may the steps of calculating the strain experience by each leg, and identifying any leg which has failed based on a very high determined resistance. Typically, the estimate of step b) is based on the step a), (e.g., the estimate is set to the mean of the measured resistances). Also, a relaxation technique may be used in step f).
A sheet-like sensor for measuring stress distribution in accordance with this invention may include a grid of members which change in resistance when subjected to strain, the members intersecting at internal nodes and intersecting at boundary nodes at the periphery of the grid defining a plurality of legs and means, such as an analyzer, connected only to the boundary nodes, for calculating any change in resistance in all of the legs based solely on the measured resistance of the legs between the boundary nodes.